A possible explanation for the lack of definitive morphological markers for BCNS OKs may relate to their aetiology: it has been observed that genetic abnormalities (mutations and loss of heterozygosity) of common tumour suppressor genes, including the drosophila-homologous Patched gene (PTCH) are associated with the BCNS (as well as some other epithelial tumours, such as basal cell carcinomas). The gene discussed is PTCH1; the disease is basal cell carcinoma.